Rotary drilling techniques are used in well drilling to reach and penetrate subterranean oil-containing formations in the earth's crust. In a typical rotary drilling technique, a drill string, having a drill bit connected to its lower end, is rotated, thereby allowing the drill bit to penetrate the earth by cutting and breaking the earth formations it contacts. A drilling fluid normally is circulated down the drill string and through ports provided in the drill bit, and back up to the surface through the annulus formed between the drill string and the wall of the well. The circulating drilling fluid performs numerous functions such as removing the cuttings from the well, cooling the bit, and applying hydrostatic pressure upon the penetrated formations to contain pressurized fluids within the formation.
Common drilling fluids are water- or oil-based liquids containing suspended solids and are normally termed "drilling muds". Whenever possible, usually for reasons of economy, water-based drilling muds are used throughout the drilling operation. The suspended solids in water-based drilling muds are typically clays from the kaolinite, montmorillonite or ilite groups. These clays impart desirable thixotropic properties to the drilling mud and also coat the walls of the well with a relatively impermeable sheath, commonly called a "filter cake", that retards fluid loss from the well into the formations penetrated by the well.
An exemplary montmorillonite clay that can be used in a water-based drilling mud is bentonite. The bentonite is dispersed within the water-based liquid as colloidal particles and imparts various degrees of thixotropy to the drilling mud. The drilling muds often contain other components to further improve the rheological properties of the drilling mud, such as increasing drilling mud density by adding barite (barium sulfate) or galena (lead sulfide).
One difficulty often encountered in rotary drilling operations involves the loss of unacceptably large amounts of the drilling mud into a porous or cracked formation penetrated by the drill. The loss of drilling mud is termed "lost circulation", and the formation is termed a "lost circulation zone" or a "thief formation". Lost circulation occurs when the well encounters a formation either having unusually high permeability or having naturally occurring fractures, fissures, porous sand formations, cracked or cavernous formations and other types of strata characterized by crevices, channels or similar types of openings conducive to drilling fluid loss. In addition, it is also possible for a formation to be fractured by the hydrostatic pressure of the drilling mud, particularly when a changeover is made to a relatively heavy mud in order to control high internal formation pressures.
When lost circulation occurs, the drilling mud pumped into the well through the drill string enters the cracks in a cracked formation or the interstices of a porous formation and escapes from the wellbore, therefore precluding return of the drilling mud to the surface. In the most severe situation, the lost circulation zone takes the drilling mud as fast as it is pumped into the wellbore, and, in less severe situations, circulation of the drilling mud can be greatly reduced, and eventually result in a shutdown of drilling operations. Normally, the maximum amount of drilling mud loss that is tolerated before changing programs is approximately one barrel per hour. If greater amounts of drilling mud is lost, corrective measures are needed. Drilling generally is not resumed until the thief formation is closed off and circulation of the drilling mud reestablished.
The interruption of normal circulation prevents the entrainment of cuttings and other materials from the borehole, leads to reduced hydrostatic pressure possibly followed by the influx into the wellbore of high pressure formation fluids, can result in the flooding of oil-producing zones with mud or the like, and may eventually cause the drill string to become stuck in the borehole. Even in situations where circulation is not completely lost and some drilling mud can return to the surface, the drilling mud flowing into the lost circulation zone must be replaced continuously. If the drilling mud loss is sufficiently high, the cost of continued drilling or well operation may become prohibitive. Therefore, the lost circulation of drilling mud is a condition that must be prevented or be corrected as quickly as possible.
The best method of controlling lost circulation is to conduct a drilling program such that mud loss will not occur. However, situations exist wherein even correct drilling techniques cannot avoid lost circuation. Therefore, many methods have been used in attempts to plug the cracks or interstices of lost circulation zones to prevent the escape of drilling muds. As a result, a wide variety of materials have been pumped into the well with the drilling mud in an effort to bridge or fill the cracks or interstices of thief formations. It has been found that some materials are successful under certain drilling conditions, yet the same material is unsuccessful under other drilling conditions.
One common method is to increase the viscosity of the drilling mud or to increase the resistance of the drilling mud to flow into the formation. Another technique involves the addition of a bulk material, such as cottonseed hulls, cork, sawdust, perlite, ground walnut shells, hay, wood shavings, granular plastic, vermiculite, rock, mica flakes, leather strips, beans, peas, rice, sponges, feathers, manure, fish scales, corn cobs, glass fiber, ashphalt, ground tires, burlap or other fabrics to the drilling mud. By adding these fibrous, flaky or granular solids to the drilling mud, and pumping the resulting mixture into the borehole, a bridge or mat forms over the cracks or interstices responsible for drilling mud escape.
Although lost circulation zones frequently are plugged by such bulk materials, successful plugging of the thief formation is not assured. Even if large volumes of a solids-containing drilling mud are pumped into the borehole, a bridge or mat may never form over the cracks or interstices of the thief formation. Moreover, the introduction of large quantities of a drilling mud containing a relatively high percentage of bulky solids can produce pressure surges that cause further fracturing and therefore result in additional fissures for even greater drilling mud losses. Bulk materials further proved unsuccessful in sealing off porous formations because they have a tendency to deteriorate under the high drilling pressures, and therefore decrease in volume and become slimy so as to "worm" into the formation openings without forming an effective seal.
Another method utilized to reduce or eliminate lost circulation is to use a cement, such as Plaster of Paris or a silicate, either alone or in combination with the previously discussed bulk materials, volcanic ash, gels and/or other similar materials. However, the cement itself often presents a problem by separating out of the cement slurry that is introduced in the well. The heavier cement particles in the slurry have a tendency to separate from the water and become dehydrated before the porous subterranean formation is sealed. Furthermore, even if the cement is admixed with ingredients designed to prevent premature dehydration, the cement slurry often passes into the porous formation without effectively plugging the openings in the wellbore sidewall. An additional problem encountered with the compositions conventionally used to reduce or eliminate lost circulation is that such compositions must be added gradually, over an extended period of time, therefore shutting down drilling operations for an extended period of time.
In order to overcome the above-mentioned deficiencies associated with the use of solid plugs, it has become a practice to attempt to plug the more porous thief formations and to stop drilling mud loss with a "soft plug", such as a gel. For example, in U.S. Pat. No. 2,800,964, Garrick teaches reducing lost circulation by placing a gel, formed by a liquid-clay dispersion, into the lost circulation zone. Kelly, in U.S. Pat. No. 3,467,208, teaches using an oleophilic bentonite to form a gel and therefore stop lost lost circulation. In U.S. Pat. No. 3,785,437, Clampett et al discloses plugging porous formations by introducing a water-soluble polymer into lost circulation zone and crosslinking the polymer in situ. However, in practical applications of the method of Clampett et al, it was difficult to control gellation characteristics of the water-soluble polymer, and therefore difficult to assure reduction of lost circulation.
A variety of compositions and methods have been proposed to reduce or eliminate lost circulation in wells. Numerous attempts have been made to find a lost circulation composition that is effective in reducing lost circulation, is economical, is easily introduced into the wellbore with minimum drilling disruption, and is stable under the high temperature and pressure conditions encountered in the wellbore. However, the prior art methods and compositions have deficiencies and drawbacks, making the continued search for an effective and efficient method and/or composition for sealing fractures and fissures encountered during drilling operations necessary.
Examples of prior art methods and compositions for controlling lost circulation include Kita et al, in U.S. Pat. No. 4,551,256, disclosing coating inorganic particles, such as bentonite, with a water-absorbing polymer, such that during excavation or drilling, a slurry is formed that swells and is not lost into the ground or cracks in the formations. McKinley et al, in U.S. Pat. No. 4,526,240, teaches that a fibrous mass, comprised of a fibrous absorbent and a water-swellable hydrophilic polymer, injected down a borehole to the lost circulation zone with an inert solvent, like kerosene, will swell and form a seal at the lost circulation zone after contact with water. Both methods have the disadvantages of being expensive and cumbersome in that the Kita method utilizes polymer-coated particles regardless of the presence or absence of thief formations, and the McKinley method utilizes an inert, non-aqueous solvent to introduce the lost circulation composition to the thief formation.
Another method of reducing lost circulation is disclosed by Delhommer et al in U.S. Pat. No. 4,633,950, wherein a hydrocarbon-absorbent polymer is dispersed in an aqueous carrier, then injected to the lost circulation zone. Once the aqueous carrier-polymer mixture is correctly placed at the lost circulation zone, a hydrocarbon is mixed with the aqueous carrier-polymer mixture to swell the hydrocarbon-absorbent polymer and seal the lost circulation zone. Similarly Walker, in U.S. Pat. No. 4,635,726, teaches dispersing a water-absorbent polymer in a hydrocarbon fluid then injecting the mixture into the lost circulation zone. Once the hydrocarbon-polymer mixture is correctly placed in the lost circulation zone, water is mixed with the hydrocarbon-polymer mixture such that the polymer absorbs water and swells to close off the lost circulation zone. Also in U.S. Pat. No. 4,664,816, Walker teaches reducing lost circulation in wellbores by introducing a water-absorbent polymer encapsulated by a protective casing to prevent the polymer from expanding by absorbing water until it reaches the lost circulation zone. Walker teaches that the protective casing can be a film or waxy substance that dissolves or melts at the desired temperature within the borehole, thereby releasing the water-absorbent polymer and allowing it to expand by absorbing water. The resulting water-swelled polymer serves to seal the lost circulation zone and thereby reduce lost circulation.
Other methods of reducing lost circulation are taught in U.S. Pat. No. 4,498,995 disclosing the use of expanded aggregates from clay, clay-shale or slate that are heat and pressure stable; U.S. Pat. No. 4,439,328 disclosing a well servicing fluid comprising an oleoginous liquid, a water-soluble polymer, an alkaline earth metal, a gellant and, optionally, a dispersant; U.S. Pat. No. 4,282,928 disclosing the introduction of discrete spheroidal microgels of a water-swollen or water-swellable crosslinked polymer into the formation; U.S. Pat. No. 4,059,552 disclosing water-swellable polymers for plugging finely porous permeable subterranean strata; U.S. Pat. No. 4,014,394 disclosing bentonite and a magnesium oxide drilling agent as a lost circulation slurry; U.S. Pat. No. 3,909,421 disclosing blending dry, powdered polyacrylamide with bentonite, adding water to the mixture, thus introducing the resulting aqueous mixture down the borehole; U.S. Pat. No. 3,724,565 disclosing an aqueous mixture of a dispersion agent in a water-dispersible oleophilic colloid as a lost circulation compound; U.S. Pat. No. 3,724,564 disclosing lost circulation control by contacting an aqueous and oleoginous liquid to form a gel and plug the thief formation; U.S. Pat. No. 2,836,555 disclosing bentonite encapsulated within a polymeric coating having a hole in the coating, then introducing the encapsulated composition down the borehole, wherein water enters the hole in the polymer coating, swells the bentonite and ruptures the coating, thereby sealing the formation. Other patents relating to lost circulation, and to methods and compositions to reduce or eliminate lost circulation include U.S. Pat. Nos. 4,261,422; 4,128,528; 3,467,208; 3,448,800; 3,198,252; 3,082,823; 3,078,920; 3,053,764; 2,683,690; and British Patent No. 869,333.
Each of the above-discussed processes has sufficient disadvantages making necessary the continued search for an effective, efficient method of reducing or eliminating lost circulation without undue interruption of the drilling process. The present invention provides such a method and composition for reducing and/or eliminating lost circulation to thief formations in wellholes. The method and composition of the present invention provide improved lost circulation control over the compositions and methods disclosed in the prior art, and can be utilized with minimal or no interruption of the drilling operation.
Surprisingly and unexpectedly, it has been found that a mixture of a water-insoluble, water-swellable polymer and bentonite, formed into pellets, can be added to a drilling mud to appreciably improve the control of lost circulation. The pellets can be introduced with the drilling mud directly down the borehole when a lost circulation zone is encountered, thereby reducing or eliminating lost circulation essentially without interrupting the drilling process. In contrast to the prior art, the composition and method of the present invention do not require swell-inhibiting techniques, such as treating the lost circulation composition with an inert, hydrophobic solvent or an oil, before introducing the lost circulation composition into the wellbore and the area of lost circulation. Additionally, the method and composition of the present invention do not require the use of bulky or insoluble fillers, such as coal or sawdust.
In accordance with the method of the present invention, the bentonite-polymer pellets can be introduced directly into the wellbore such that the pellets can pass through the wellbore, while in contact with an aqueous drilling mud, to reach the area of lost circulation in a substantially unswelled state. At the area of lost circulation, the pellets can accumulate, absorb water and swell to form an essentially fluid-tight plug, thereby sealing the porous formation and preventing further drilling mud loss. By employing the method and composition of the present invention, improved drilling performance and lost circulation control efficiency are realized due to superior plugging of the thief formation without major drilling interruptions.
Both bentonite and water-swellable polymers have been used to control lost circulation. However, the combination of bentonite and a water-swellable polymer, in pellet form, provides a method and composition for lost circulation control that is new in the art and provides unexpectedly improved results in controlling lost circulation. These and other advantages of the present invention will be described more fully hereinafter.